This invention relates to systems and methods for calibrating a charging unit of an image forming apparatus.
In electrophotographic printing, a photoconductive surface, often a photoconductive belt, is charged by a charging unit and then selectively exposed to image data to selectively discharge portions of the charged photoconductive surface. This forms a latent electrostatic image on the photoconductive surface. Charged toner material is applied to the latent image bearing portion of the photoconductive surface to convert the latent electrostatic image into a developed image. Finally, the developed, or toner, image is transferred to a sheet of recording medium, such as paper, by charging the backside of the paper with another charging unit to attract the toner of the developed image from the photoconductive surface to the paper. The toner of the developed image is then at least semi-permanently fixed to the sheet of recording material, such as, for example, by heating a thermoplastic toner material to fuse the toner material to the sheet of recording material. An example of this process is more fully described in U.S. Pat. No. 2,297,691 incorporated herein by reference in its entirety.
The device that performs charging of the photoconductive surface and the recording medium may be a dicorotron, which may include an insulated coronode disposed adjacent to a conductive shield and photoconductive surface. The insulated coronode may be driven by an AC signal at a voltage high enough to create a corona plasma in the area surrounding the insulated coronode. When a bias potential is applied to the conductive shield relative to the photoconductive surface, charged particles may flow through the plasma and may be applied to the photoconductive surface. The amount of charge flowing through the plasma and deposited on the photoconductive surface may thus depend on the bias voltage between the conductive shield and the photoconductive surface, as well as on the AC voltage applied to the insulated coronode.
Charge devices such as dicorotrons, scorotrons (which include a conductive grid between the coronode and the photoconductive surface) and corotrons may be located at various places along the path of the photoconductive belt through the image forming device. In particular, the charging units may be located before the exposure units of each color station in a color image forming device, as well as before the transfer station where the toner particles are transferred to the recording medium.